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Long-Night Moon

Santa shouldn’t need Rudolph’s help tonight, because the full Moon will light up the Christmas Eve sky. In fact, there’s more moonlight tonight and tomorrow night than at any other time of the year.

The Moon is full before sunrise tomorrow. It lines up opposite the Sun in our sky, so it does the opposite of what the Sun does in the daytime sky. Since the winter solstice — the shortest day of the year in the northern hemisphere — took place just three days ago, the Sun is still putting in its most feeble appearances of the year. It rises late, sets early, and scoots low across the south during the short day.

So the full Moon does just the opposite: It rises around sunset, climbs high across the sky during the night, and sets around sunrise.

The difference in sunlight and moonlight is more dramatic at high northerly latitudes. Seattle, for example, will see just eight-and-a-half hours of sunlight tomorrow, followed by more than 15 hours of moonlight.

Not only does the Moon remain in view for a long time, it also climbs highest across the sky. That’s because the Moon lies close to the ecliptic — the Sun’s path across the sky. At this time of year, the ecliptic arcs low across the sky during the day because Earth’s north pole is tipped away from the Sun. But the full Moon is on the opposite side of Earth, so the north pole tips toward it. As a result, the ecliptic climbs high across the sky — and so does the full Moon.

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